1. Field of the Invention
The present invention relates to a method and apparatus for determining a status of a laser gas mixture within a discharge chamber of a gas discharge laser, and more particularly for measuring a parameter of the output laser beam versus the applied driving discharge voltage or other adjustable input parameter and comparing the measured data with a master set or sets of stored data to determine the status of the laser gas mixture and/or whether any electrical, mechanical or optical problems exist within the system.
2. Discussion of the Related Art
Gas discharge lasers, e.g., excimer or molecular lasers, are well known as valuable tools for many industrial applications. There is a great desire to have precise control and simultaneous stabilization of many laser parameters over extended durations of operation, especially with regard to excimer laser applications in many fields including electronics and photolithographic processing. The amount of xe2x80x9cup timexe2x80x9d of a laser, or time when a laser is in operation and being used for industrial application, is a key variable in operation cost considerations. It is desired to be able to successively adjust, sensitively control and carefully stabilize various laser parameters efficiently and simultaneously.
The type and quality of the gas discharge affects many significant laser parameters such as output power, energy stability, efficiency, bandwidth, long and short axial beam profiles, temporal and spatial pulse width, and beam divergence and coherence. The quality of the gas discharge depends on such factors as the composition of the gas mixture in the discharge chamber, the quality of preionization used, properties of the discharge circuit, and profiles of the electrodes used. See R. S. Taylor, Appl. Phys. B41, 1-24 (1986). Decomposition and contamination of the gas mixture and the design of the gas exchange system (e.g., flow speed) also strongly determine the limits of achievable laser parameters. A fast gas exchange between electrodes may be realized by using a laser discharge chamber design including fast blower gas circulation. Cryogenic and electrostatic equipment and techniques may be used for additional gas purification. See German Patent No. 32 12 928.
Optimum gas mixtures for various gas discharge lasers are generally known. A partition of 0.1:1.0:98.9 F2:Kr:Ne is thought to be substantially optimal for a KrF-excimer laser, for example, and 0.1:99.9 F2:Ne for an F2 laser. FIG. 1 shows a plot of laser output power versus F2 concentration and represents a way of determining what the optimal F2 concentration actually is. As time goes by and the laser is operated, the gas mixture degrades or xe2x80x9cagesxe2x80x9d continuously resulting in a dilution of F2 and a consumption of F2 via chemical reactions with metal dust. In this regard, U.S. Pat. No. 4,977,573 to Bittenson et al., which is assigned to the same assignee as the present application, is incorporated herein by reference. After a parameter changes by a certain amount, such as after a certain amount of time, number of pulses or change in discharge voltage compensating the gas deterioration to maintain constant output energy, among others, a replenishment such as halogen injection (HI) or partial gas replacement (PGR) of a certain amount of the gas mixture or an entirely new fill of the gas mixture is performed to, as nearly as possible, substantially reinstate the original partition of the gas and optimize laser parameters.
It is desired to be able to prolong the lifetime of the laser gas mixture. It is further desired to have suitable measuring tools that indicate when and to what extent the laser gas mixture is aged before problems associated with laser parameters varying from optimum as the laser gas degrades lead to significant reductions in laser output performance, processing errors and excessive laser downtime.
A mass spectrometer may be used for precision analysis of the composition of the gas mixture. See U.S. Pat. No. 5,090,020 to Bedwell. However, a mass spectrometer is an undesirably hefty and costly piece of equipment to incorporate into a continuously operating excimer or molecular laser system. Other ways of monitoring the status of a laser gas mixture include measuring a spectrum width or bandwidth of a laser emission (see U.S. Pat. No. 5,450,436 to Mizoguchi et al.), measuring a beam profile of the laser emission (see U.S. Pat. No. 5,642,374 to Wakabayashi et al.), and measuring other characteristics such as the width of the discharge or temporal pulse width of the output beam wherein a rough estimate of the status of the gas mixture may be made. See U.S. Pat. No. 5,440,578 to Sandstrom. Another known technique of measuring the age of the laser gas mixture is to count the total number of laser pulses from the most recent new fill of the discharge chamber. See U.S. Pat. No. 5,646,954 to Das et al.
A number of techniques are known wherein the output beam energy or efficiency is monitored and steps are taken to maintain the output beam at an optimum energy. See U.S. Pat. No. 3,899,750 to Hochuli, U.S. Pat. No. 4,429,392 to Yoshida et al., and U.S. Pat. No. 4,977,573 to Bittenson et al. Rare and halogen gas concentrations have also been maintained by using a complex series of chemical reactions to determine the gas mixture concentrations and replenish depleted gases as needed. See U.S. Pat. No. 4,740,982 to Hakuta et al.
The above parameters measured and monitored for determining the status of the laser gas mixture are each dependent on other parameters in addition to the gas mixture status, e.g., stabilized output energy, repetition rate, etc. They are based on generally known behaviors of laser systems and general experience regarding gas mixture aging in discharge chambers. It is desired to have a technique for monitoring the gas mixture status without variations in other parameters affecting the analysis. It is also desired that properties of the discharge chamber, optics and discharge circuit, among others, be taken into account in a gas mixture status monitoring procedure to provide greater completeness and accuracy.
An effective and sensitive method and apparatus for determining a status of a gas mixture and its degree of aging or degradation is provided by the present invention. An internal computer control system determines a status of a laser gas mixture in a discharge chamber preferably during a special check sub-routine. An output beam parameter, e.g., pulse energy, bandwidth, long or short axial beam profile, energy stability, energy efficiency, amplified spontaneous emission (ASE), discharge width, beam divergence, beam coherence, spatial pulse width or temporal pulse width, versus an input parameter, e.g., driving voltage, are measured and stored by an internal control system. The measured data are then compared with a master profile or master set of data measured when optimal gas mixture conditions exist such as after a new fill of the discharge chamber with the gas mixture. Preferably, multiple master data sets are measured each corresponding to different operating conditions of the laser separate from gas mixture status.
A method is provided for determining the status of a gas mixture of a gas laser system which generates an output beam having at least one characteristic parameter that is measurable, e.g., output pulse energy, bandwidth, long or short axial beam profile, energy stability, energy efficiency, amplified spontaneous emission (ASE), discharge width, beam divergence, beam coherence, spatial pulse width or temporal pulse width, within the system and has a discharge chamber containing a gas mixture within which energy is supplied to the gas mixture by a power supply via application of a driving discharge voltage to electrodes of a discharge circuit. A master profile or master data set of the output beam parameter is measured versus an input parameter such as driving voltage for a laser having specified operating conditions preferably including an optimal gas mixture depending on the operating conditions. This master data set is stored into the memory of the control system of the laser for which it is desired to measure the status of the gas mixture at later times. At another time, preferably during a check sub-routine such as during a start-up procedure or during each start-up procedure, a current status data set of the output beam parameter measured in the master data set versus the input parameter measured in the master data set is measured for the gas discharge laser. The current status data set is then compared with the master data set and deviations are noted, such as in the values and/or derivatives, e.g., slopes, or integrals at data points along the curves defined by the data sets.
A small deviation, e.g., of value or slope, between the master and current status data sets will typically indicate that the laser gas mixture has aged or that the F2 concentration of the gas mixture is somewhat depleted. A large deviation may indicate that a complete new fill is necessary or that serious mechanical, electrical or optical hardware problems are present in the system. A follow-up procedure is then preferably performed depending on whether and what type of deviation exists between the master and current status data sets.
A current data set may be measured after a new fill, and the data set compared with the master data set obtained after a previous new fill. When deviations are present between the master and current data sets just after a new fill, assuming operating conditions haven""t otherwise been changed, then hardware problems are suspected, as aging of the gas mixture will have not yet occurred.
In the preferred embodiment, one or more calibration data sets are initially measured and stored corresponding to different gas mixture statuses and/or operating conditions of the laser. A processor compares a current status data set which is deviated from the master data set with one or more of the calibration data sets to find the calibration data set which is most similarly deviated from the master data set. The processor then determines, from stored gas mixture data regarding the similarly deviated calibration data set, what type of follow-up procedure should be performed. For example, a follow-up procedure may include replenishment of a certain amount of a molecule including an active halide species such as F2, or HCl, an active rare gas or a combination of a molecular halide and an active rare gas.
A gas discharge laser system is also provided. The laser system includes a discharge chamber containing a laser gas mixture and a resonator for generating a laser beam. A power supply circuit delivers energy to the gas mixture by providing a driving voltage to a discharge circuit that ultimately produces a potential difference across electrodes in the discharge chamber. The system further includes means for measuring an input parameter, e.g., the driving voltage, and means for measuring an output beam parameter which varies with the status of the gas mixture, such as one of those mentioned above, e.g., the output power of the laser beam. A processor then receives the input and output parameter measurements as a current status data set. The current status data set is compared with a master data set that represents a substantially optimal gas mixture and a laser operation status. The master data set used may differ between laser systems of varying ages or other operating conditions such as repetition rate. The current status data set may also be compared with one or more calibration data sets, such as were measured, e.g., following one or more halogen replenishment procedures, during an initial start-up procedure, during one or more subsequent start-up procedures, after subsequent new fills or under other than optimal gas mixture conditions. Follow-up procedures may be performed depending on the types of deviations, such as values, derivatives or integrals at points along curves defined by the data sets, noted from the comparison.